Taking advantage of their excellent resiliency, flexible urethane foams have been used as cushioning and back-rest materials in a broad range of applications, such as furniture, bedding, car upholstery and so on. According to the production processes used, such urethane foams are roughly divided into slab foams and mold foams.
A slab foam is available in the shape of a block produced by foaming under no restraint and pieces of the desired shape are cut out from the block for use. A mold foam is a shaped article produced by foaming in a metal or plastic mold. Mold foams are mostly used as automotive parts.
The production technology for such flexible urethane mold foams is generally divided into the cold cure process and the hot cure process. Both processes have their own advantages and disadvantages. Thus, the mold foam produced by the cold cure process is generally known as HR (high resilience) foam and features a high resilience and a large SAG coefficient, for instance, thus being very desirable in physical characteristics. Moreover, this foam can be cured at low temperature in a short cure time as an additional advantage. It is further advantageous in that the foam yield is high and that the foam hardly cracks or shrinks. However, the applications of the foam produced by this process are limited to high-density cushions because reducing the foam density results in drastic aggravation of the humid age compression set.
On the other hand, the hot cure process is disadvantageous in that it requires not only a high curing temperature but also a long cure time and due to a variation in the amount of the catalyst and fluctuations of mold temperature and depending on mold geometry, defects such as cracks, shrinkage and loose skin are liable to develop in the product foam. Moreover, the product yield is also poor. However, the hot cure process is superior to the cold cure process in that the former enables the production of a low-density foam improved in compression set. Therefore, among the flexible urethane foams produced by the hot cure method, low-density foams are generally used as back-rest materials and medium- to high-density foams as cushioning materials.
Thus, the density and hardness of flexible urethane foams should be controlled according to intended applications.
It is common practice to use CFC-11 (trichlorofluorocarbon), which is a controlled chlorofluorocarbon, for inhibiting scorching and avoiding the risk of a fire or for implementing a low degree of hardness in the production of slab foams with a density of not more than 22 kg/cm.sup.3 or for controlling the hardness (realizing a low hardness value) and for implementing a low foam density in the production of foams by the hot cure process for use as automotive seat back-rest materials.
However, the recent control over the use of chlorofluorocarbons for the protection of the ozone layer is expected to become more and more stringent and ultimately lead to a complete ban on their use. In view of the imminent complete ban, the development of a technology for producing a low-density, low-hardness urethane foam without employing a chlorofluorocarbon is an urgent task to be tackled. The approaches so far made to this end generally comprise a switchover from CFC-11 to CH.sub.2 Cl.sub.2 and the use of an increased amount of water in the formulation. On the other hand, as to mold foams, the polyol is modified and the amount of water in the batch formula is increased to reduce chlorofluorocarbon requirements. Regarding the hot cure process, technologies for producing a low-density, low-hardness urethane foam which comprise using water alone as the blowing agent and increasing the pouring temperature beyond the conventional level have been disclosed in Japanese Tokkyo Kokai Koho H-3-176110, H-3-192109, H-2-11614 and H-3-3689, among others. However, unlike the case using a chlorofluorocarbon, it is difficult to produce a low-hardness flexible foam having satisfactory physical characteristics by using water alone as the blowing agent. To overcome this difficulty, a method employing a monool or diol-based polyoxyalkylene polyol as part of the polyol component has been proposed but this method has the drawback that the humid age compression set is increased and other physical properties are also sacrificed.
However, increasing the amount of water in the formulation causes an increased evolution of carbon dioxide gas according to the reaction --NCO+H.sub.2 O.fwdarw..about.NH.sub.2 +CO.sub.2 and although the CO.sub.2 gas contributes to foaming, of course, it encourages the crosslinking reaction as follows. ##STR1## This crosslinking reaction and hydrogen bonding between the resulting urea bonds unavoidably increase the hardness of the product foam. Therefore, when a urethane foam of a given density is produced by increasing the amount of water instead of using CFC-11, a substantial increase occurs in the hardness of the foam. Furthermore, the use of water in an increased quantity adversely affects physical properties including compression set in a considerable measure. Modifying the polyol may result in some improvement in compression set but be scarcely effective in controlling the increase of hardness.
When the urethane foam is intended for use as the back rest of a car seat, such an increased hardness is definitely unacceptable. Since a reduction in chlorofluorocarbon consumption is an urgent requirement today, a certain increase in hardness is tolerated today but the demand for reduced hardness is persistent.
It is true that the following methods for reducing the hardness of foams have been known for years. One of the methods comprises lowering the NCO index (isocyanate indicator) [the equivalent number of isocyanate groups per 100 active hydrogen atoms; --NCO and --OH/H.sub.2 O are in the ratio of 1:1 at the NCO index number of 100] of the reaction system and the other comprises adding a monohydric or dihydric alcohol to lower the functionality of the polyol component.
However, these known methods have major disadvantages such as poor moldability, undercure and poor physical characteristics of the foam (particularly humid age compression set).
On the other hand, by virtue of their satisfactory physical properties, HR foams are meeting an increasing portion of the demand but its greatest drawback is that this kind of foam cannot be easily reduced in density as compared with the hot cure mold foam.
It is, therefore, a further object of the present invention to provide a process for producing an HR foam of low density and low hardness without sacrificing the physical properties, particularly humid age compression set, of the foam and with acceptable moldability.
Among the HR formulation recently proposed, there is the formula called "all-MDI formula" employing some special MDI.
This type of formula offers a number of advantages such as high cure rate, high durability, ease of varying hardness, etc. and the demand for the foams produced from this type of formula is increasing.
However, this type of formula is hardly conducive to hardness reduction in the absence of CFC-11 and, in the domestic market, is claiming only a limited segment of the market, typically the head rest market, where foams of comparatively high density are still acceptable.
The all-MDI foam cannot be reduced in density to the extent that can be obtained with TDI-80 and TM-20 (TDI-80/polymeric MDI=80/20) but is also required to be supplied in the low density range.
On a laboratory scale, it is not impossible to produce a low-density HR foam by no more than increasing the amount of blowing agent water. However, the humid age compression set characteristic of the foam deteriorates drastically in proportion to the increasing amount of H.sub.2 O, with an associated aggravation of foam moldability. In other words, such a system is not suited for the production of foams of intricate design.
The object of the present invention is to solve the above-mentioned problems associated with the production of low-density, low-hardness flexible urethane foams.
More particularly, the object of the invention is to provide a process for producing a low-density, low-hardness flexible urethane foam without employing a controlled chlorofluorocarbon and, instead, using water substantially alone as a blowing agent. The further primary object of the invention is to create a novel polyether polyol for the establishment of an environment-friendly production technology and provide a novel process for producing a flexible urethane foam starting with such a specially created polyether polyol as part of the polyol component, which process does not necessarily require the use of a chlorofluorocarbon.